Structural panels having reinforcing cores are used in a variety of applications in the aerospace industry. Reinforcing cores are often necessary in large panels to provide sufficient rigidity and structural strength in the panel so that the panel can withstand substantial loading.
Typical panels have a bottom composite layer manufactured from a fibrous material which is impregnated with a resin. A core material of amorphous foam or honeycomb structure is then positioned on the bottom layer. A top composite layer is then bonded to the core. An edge layer is then applied to connect the top and bottom layers and to seal the core therebetween. This assembly is then cured in an autoclave at elevated temperatures and pressures.
The reinforcing core is typically designed to have substantial compression resistance along a core axis. The core is typically oriented with respect to the top and bottom layers so that the core axis is perpendicular thereto. In this way, the structural panel undergoes loading in a fashion similar to a beam wherein the top and bottom layers correspond to the flange portions of a beam and the core corresponds to the web portion of a beam. The core itself has little resistance to compression in a direction perpendicular to the core axis. Thus, the edges of the core are particularly susceptible to deformation when the panels are autoclaved.
A number of construction techniques have been adopted to prevent deformation of the core edges when the panel is autoclaved. In one technique, a peripheral edge material having substantial rigidity in a direction both parallel to and perpendicular to the core axis is positioned around the periphery of the core. In this way, pressure exerted on the edge of the panel is not transferred to the core. This method disadvantageously increases the weight of the panel.
In another technique shown in FIG. 5 of U.S. Pat. No. 4,256,790 issued to Lackman et al., the periphery of the core is tapered at an angle of less than approximately 20.degree.. The top layer of composite material covers this tapered core portion and provides a pressure surface which forms an acute angle with respect to the core axes of at least 70.degree.. It has been found that for many core materials, under typical autoclave pressure, the core will not crush if the pressure surface which transfers force to the core forms an angle of at least 70.degree. to 90.degree. with respect to the core axis. This technique incurs no weight penalty whatsoever and prevents the core edges from being crushed during autoclaving. However, this technique is disadvantageous if the core has a thickness substantially greater than one inch. For example, if the core has a thickness of approximately four inches, the tapered section will have a length of approximately twelve inches. This may be particularly inconvenient if fittings such as latches, hinges, etc. are to be attached to the panel edges.
Therefore, it is desirable for assembly purposes that tapered structural panels have edges which taper at relatively steep angles. Various prior art techniques are available to achieve steeper edged taper angles. In one technique doubler layers are interpositioned within the top and bottom layers adjacent to the tapered core portion to increase the rigidity of the core edges. In another technique, a denser core material is used on the taper periphery of the panel. In yet another technique the tapered core portion is filled with a potting material. Each of these techniques involves a weight penalty which is undesirable.
Therefore, a need exists for a structural panel construction technique which permits relatively steep taper angles on the edges of a structural panel and which does not substantially increase the weight of the panel.